We found that Muse Mammary Tumor Virus, a betaretrovirus spread through the milk, requires commensal microbiota for successful transmission. The virus exploits bacterially-produced lipopolysaccharide to elicit immunosuppressive cytokines and thus, counteract anti-virus immune response. Our studies will uncover the rules of retrovirus-microbiota-host interaction by using genetically defined and germ-free mice. PUBLIC HEALTH RELEVANCE: Most viral pathogens launch acute infections, whereby the virus replicates rapidly and disseminates to another organism prior to immune clearance or death of the host. In contrast, some viruses are able to establish persistent infections through adoption of complex relationships with their hosts and manipulation of a wide array of cellular mechanisms for their own advantage. Even though persistent viruses have evolved distinct mechanisms to enable long-term survival in the host, they all share a common trait-the ability to evade the immune system. These viruses are often transmitted most efficiently through mucosal surfaces rich in microbiota, as in the case of Mouse Mammary Tumor Virus. Here, we find that MMTV, when ingested by newborn mice, stimulates unresponsiveness towards viral antigens. This unresponsiveness alleviates immunity against the virus and allows for its indefinite persistence. This process requires intestinal microbiota, as antibiotic-treated mice or germ-free mice do not transmit infectious virus to their offspring. The MMTV-induced tolerance pathway involves activation of Toll-like receptor 4 by lipopolysaccharide and subsequent IL-6- dependent production of the inhibitory cytokine IL-10. Thus, MMTV has evolved to rely on the interaction with omnipresent microbiota to induce the neonatal oral tolerance pathway delineated in this study. Together, these findings underlie the fundamental importance of commensal microbiota in viral infections. Undoubtedly, similar mechanisms operate upon infection with viruses of different families or other pathogens that spread via the gastrointestinal route, making this application broadly significant. We propose to further investigate the role of innate immune and adaptive responses in retrovirus transmission and expand our studies to another animal model in which retroviral transmission could be both blood borne and oral. The overall objective of this proposal is to identify the adaptive immune mechanisms, which control retroviruses and to define the mechanism by which retroviruses evade these responses.